Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Heat recovery in absorption and desorption processes with reduced heat
exchange surface
[0001] The present invention relates to an economical process for the
removal of cornpo-
nents to be separated from technical gases in absorption and desorption
processes.
[0002] Such technical gases are mostly natural gas or synthesis gas, the
synthesis gas
being generated from fossil raw materials such as crude oil or coals and from
biological raw
materials. Natural gas and synthesis gas contain useful valuable gases but
also interfering
components, such as sulphur compounds, in particular sulphur dioxide, carbon
dioxide and oth-
er components to be separated such as hydrogen cyanide and water vapour.
Beside natural
gas and synthesis gas, flue gases from an incineration of fossil fuels are
also included in the
group of technical gases from which interfering components as, for example,
carbon dioxide,
are removed. The components to be separated may also be useful gases which are
to be sepa-
rated for a specific purpose.
[0003] Both physical and chemical absorbents can be used for absorption.
Chemically act-
ing absorbents are, for example, aqueous amine solutions, alkali salt
solutions, etc. Selexol,
propylene carbonate, N-methyl-pyrrolidone, morphysorb, methanol, etc. are
physical absor-
bents.
[0004] It is known from prior art to remove components to be separated from
technical gas-
es in circuit-operated absorption and desorption processes. The components to
be separated
are absorbed in the absorption device by the liquid absorbent. The gas which
is insoluble in the
solvent leaves the absorption device at the top, whereas the components to be
separated re-
main in dissolved state in the liquid absorbent and leave the absorption
device at the bottom.
Before the laden solution is fed to the top of the desorption device, the
laden solution is usually
pre-heated by heat exchange with the hot, desorbed solution, by which part of
the energy re-
quired for the desorption in the desorption device is recovered.
[0005] By means of a heating agent, a reboiler at the bottom of the
desorption device
serves to generate steam by partial evaporation of the solvent at the bottom
inside the desorp-
tion device. Here, the generated steam serves as stripping agent to remove the
components to
be separated from the laden solution. The laden solution is freed by the
stripping agent in coun-
tercurrent from the absorbed components to be separated. The stripped
components to be sep-
arated leave the desorption device at the top, with the steam portion of the
stripping agent being
condensed in a head condenser and returned to the desorption device. The
desorbed solution
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which has been freed from the components to be separated leaves the desorption
device at the
bottom, with the solution usually being cooled after heat exchange has been
carried out and
returned to the top of the absorption device. This concludes the circuit of
the absorption and the
desorption process.
[0006] DE 10 2005 004 948 B3 discloses a process for increasing the
selectivity of physi-
cally acting solvents in an absorption of gas components from technical gases.
A process for
removing sour gas components, water and aromatic and higher aliphatic
hydrocarbons as com-
pletely as possible and regenerating the absorbent as completely as possible
is described in DE
199 45 326 B4.
[0007] On account of the increasing demand for resources an economical mode
of opera-
tion in all fields has long become an important basis for the further
development. The aim is
therefore to make the absorption and desorption process as efficient and cost-
effective as pos-
sible.
[0008] In the absorption, which in most cases is carried out at a working
pressure of 1 to
100 bar, an absorption temperature of 20 C to up to 70 C has proved to be
advantageous for
removing the components to be separated from the technical gas.
[0009] The temperature required for a desorption in a desorption device is
generally higher
than that in the absorption device. Usually the desorption device is operated
at a temperature of
80 C to up to 140 C and an absolute pressure of 0.2 to up to 3 bar.
[0010] Energy saving can be achieved by utilising the waste heat of the
streams through
the absorption and desorption process in an efficient manner. Before the laden
solution dis-
charged from the absorption device is fed to the desorption device for
regeneration, the laden
solution is, for example, pre-heated by means of the hot solution leaving the
desorption device,
in order to bring the temperature of the laden solution closer to the
temperature required for
desorption. The separated components from the desorption device are cooled to
recover the
stripping vapours as condensate and to allow their further processing. In
practice this has hith-
erto been done by a condenser. As in EP 1 569 739 B1, the exhaust steam rising
after stripping
is cooled by a condenser in the desorption device using hydrogen sulphide-
containing cooling
water.
[0011] The regenerated solution leaves the desorption device at the bottom
at a ternpera-
ture of usually at least 100 C. Before the regenerated solution can
subsequently be returned to
the absorption device, the solution is to be cooled down to a temperature of
20 C to 70 C. By
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means of the heat exchanger heat is transferred from the hot, regenerated
solution to the cold,
laden solution. Maximum temperature approximation between the hot, regenerated
solution
entering the heat exchanger and the pre-heated laden solution leaving the heat
exchanger will
allow a correspondingly high recovery, obtaining the heat contained in the
solution stream leav-
ing the desorption device. This temperature approximation usually amounts to
approx. 10 K.
Such a high temperature approximation requires a correspondingly large heat
exchange surface
incurring correspondingly high costs. Therefore a temperature approximation of
below 10 K for
the recovery of the heat level of the desorption device is not acceptable any
more for economi-
cal reasons.
[0012] EP 1 606 041 B1 discloses a method for the selective removal of sour
gas compo-
nents from natural gas or synthesis gas, with the sour gas components being
removed selec-
tively within two absorption stages to achieve an economical mode of
operation.
[0013] By heat exchange between the stream to be heated and the stream to
be cooled the
waste heat produced in the absorption and desorption process circuit is
recovered. This heat
exchange has two effects: The fluid to be cooled transfers its heat to the
fluid to be heated. In
this way, the heat energy available in the process circuit is recovered
without requiring addition-
al energy from external sources.
[0014] The aim of the invention therefore is to provide an economically
improved process
including heat recovery by reduced heat exchange surface as compared to prior
art, the process
being used for the removal of components to be separated from technical gases
in absorption
and desorption processes.
[0015] The aim is achieved by a process for the removal of components to be
separated
from technical gases, with the process being implemented by means of
absorption and desorp-
tion processes using liquid absorbents, in which at least one absorption
device (20) is provided,
which includes at least one mass transfer section where the components to be
separated are
absorbed by the liquid absorbent, and at least one desorption device (22) is
provided, with the
desorption device (22) comprising at least one heat transfer section (22a), a
stripping section
(22b) and a reboiler (8) at the bottom, with the heat transfer section (22a)
being located above
te stripping section (22b) and the temperature in the desorption device (22)
being higher than
the temperature in the absorption device (20).
[0016] The solution laden with the components to be separated is heated by
a heat ex-
changer before this solution is fed to the desorption device (22). The
remainder of the energy
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required by the desorption is supplied by the reboiler (8) at the bottom of
the desorption device
(22). The components to be separated, which have been stripped off by the
stripping agent,
leave the top of the stripping section (22b) as exhaust steam, which is then
introduced into the
heat transfer section (22a), cooled accordingly and leaves the desorption
device (22) at the top.
The solution which, after desorption, is free of the components to be
separated leaves the de-
sorption device (22) at the bottom and, after heat exchange and cooling, is
returned to the top of
the absorption device (20).
[0017] At least part of the laden solution leaving the absorption device
(20) is branched off
before being heated by a heat exchanger and fed to the top of the heat
transfer section (22a).
This laden part-stream is heated by the steam rising from the bottom part of
the desorption de-
vice (22b) via heat exchange in the heat transfer section (22a). The residual
stream of cold,
laden solution leaving the absorption device (20) is pre-heated via heat
exchange with the hot,
regenerated solution leaving the desorption device (22), with the heat
exchange being config-
ured such that the total heat exchange surface required for the absorption and
desorption pro-
cess is reduced.
[0018] It goes without saying that it is possible to feed all of the laden
solution leaving the
absorption device (20) non-branched to the top of the heat transfer section
(22a) for heating.
[0019] Heating the laden solution via the heat transfer section (22a) at
the top of the de-
sorption device increases the temperature of the stream as compared to the
same stream up-
stream of the branch. This results in a considerably higher mean logarithmic
temperature differ-
ence for the heat exchanger than in prior art, which leads to a
correspondingly significantly re-
duced heat exchange surface for the heat exchanger. With regard to the total
heat exchange
surface required for the entire absorption and desorption process there is a
likewise significant
reduction in the overall required heat exchange surface.
[0020] Heating via the heat transfer section (22a) may take place by direct
or indirect heat
transfer. The exhaust steam rising from the stripping section (22b) transfers
its heat to the laden
solution to be heated. In the case of direct heat transfer, the heat transfer
section (22a) is pro-
vided with a mass transfer section, which is equipped with mass-transfer
elements where direct
heat transfer is implemented, the mass-transfer elements meaning the internals
of a column
used for heat and mass exchange, such as packing material, structured
packings, trays (bubble,
valve, sieve trays). The laden solution which trickles downwards absorbs the
heat from the ris-
ing exhaust steam while the exhaust steam is being cooled accordingly. In the
case of indirect
heat transfer, the heat transfer section (22a) can be provided with a
condenser in which indirect
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heat transfer is implemented. The condenser on the one hand cools the rising
exhaust steam as
required and on the other hand heats the laden solution to be heated as
desired.
[0021] After the part-stream has been pre-heated in the heat transfer
section (22a), the pre-
heated part-stream is passed on to the stripping section (22b), or the pre-
heated part-stream is
withdrawn below the heat transfer section (22a), merged with the cold residual
stream (5a, 5b)
leaving the absorption device (20), further heated via a heat exchanger (21)
by means of the
hot, regenerated solution leaving the desorption device (22), then being fed
to the stripping sec-
tion (22b). In another advantageous embodiment, the pre-heated part-stream is
withdrawn be-
low the heat transfer section (22a), merged with the pre-heated residual
stream of the solution,
and further heated via another heat exchanger by means of the hot, regenerated
solution leav-
ing the desorption device (22), then being fed to the stripping section (22b).
[0022] This process can be run with a physically or a chemically acting
absorbent. The pro-
cess can be used in particular for the removal of sour-gas components from
technical gases.
[0023] Fig. 1 represents the state of the art.
[0024] Fig. 3 represents an alternative mode of operation embodying the
invention, accord-
ing to which the stream which has been pre-heated in the heat transfer section
(22a) is com-
pletely routed to the top of the stripping section (22b).
[0025] The mode of operation embodying the invention is illustrated herein
below by pro-
cess flow diagram Fig. 2.
[0026] In the absorption and desorption process circuit the solution laden
with components
to be separated leaves the absorption device at the bottom and is fed to the
desorption device
(22) for regeneration/desorption, with the desorption device (22) comprising
at least one heat
transfer section (22a) with mass-transfer elements/condenser, a stripping
section (22b) and a
bottom reboiler (8). The bottom reboiler serves to heat the stripping agent
for the stripping of the
components to be separated from the laden solution in the stripping section
(22b).
[0027] The cold, laden solution leaving the absorption device (3) is
branched off before
being heated, part of it (4) is fed to the top of the heat transfer section
(22a), the remainder (5a)
is merged with the pre-heated part-stream and further heated via a heat
exchanger (21).
[0028] The cold, laden solvent stream (4) fed to the heat transfer section
(22a) makes the
stripping steam rising from the bottom cool down and condense. In this way,
practically all of the
heat of the stripping steam is directly or indirectly transferred to the
solution trickling down from
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the top. The cooled steam (13) entraining the components to be separated
leaves the top of the
desorption device at a temperature which is approximately as that of the laden
solution (4) when
entering the heat transfer section (22a). High temperature approximation
between the steam
leaving the top (13) and the laden solution (4) supplied is achieved by the
direct/indirect heat
and mass transfer in the heat transfer section (22a).
[0029] Via a chimney tray below the heat transfer section (22a), the pre-
heated solution
(4a) is withdrawn below the heat transfer section (22a), merged with the
residual stream (5a)
and fed to the heat exchanger (21) in order to further increase the
temperature of the stream
thus merged. At the same time the solution that has already been regenerated
(9,10) flows
through the same heat exchanger (21) and is thus cooled. By the pre-heating in
the heat trans-
fer section (22a), the mean logarithmic temperature difference between the two
solutions has
become greater.
[0030] A comparison of Fig. 1 and Fig. 2 shows that the head condenser (18)
which fre-
quently consists of high-quality material, the reflux drum(19) and the reflux
pump (15) are omit-
ted in Fig. 1. The heat exchange surface of the heat exchanger (21) is
considerably smaller than
before on account of the increased mean logarithmic temperature difference and
the less heat
to be transferred. At the same time the heat exchange surface of the heat
exchanger (17) is
larger than before to allow cooling the regenerated solution (12) back to
absorption temperature
but all in all the result is significantly better than the result according to
prior art.
[0031] Fig. 3 illustrates another variant. The difference in comparison to
Fig. 2 is that the
stream which has been pre-heated in the heat transfer section (22a) is not
withdrawn from the
desorption device but further supplied to the top of the stripping section
(22b).
[0032] Below, a simulation example aiming at the removal of the interfering
sour-gas com-
ponents hydrogen sulphide and carbon dioxide from the synthesis gas is to show
the differ-
ences in the processes in tables 1, 2 and 3 in a clear manner.
[0033] By means of the parameters temperature, heat exchange surface and
thermal out-
put, the heat recovery of an absorption and desorption process according to
prior art is com-
pared with that of the invention. In this comparison, all heat exchangers are
assumed to be
shell-and-tube heat exchangers.
LMTD: mean logarithmic temperature difference
<Kw>: cooling water
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<ND>: low-pressure steam
WT: heat exchanger
Table 1: Total exchange surface of an absorption and desorption process
according to prior art.
Heat ex- Heat output Stream Temp
Temp Stream Surface/
changer no. [kW] no. [ C] [ C} no.
LMTD WT [m2]
21 104288.1 11. 46.1 <-- 125.9
10. 7.5 27794
3. 41.2 --> 115 6.
18 17420.7 13. 108 --> 50
14. 40.8 1710
<Kw> 40 <-- 19 <Kw>
17 14516.7 11. 46.1 --> 35 12. 7.5
1948
<Kw> 37 <-- 19 <Kw>
8 34100 7. 125.8 --> 125.9 <> 21.3 1599
<ND> 152.0 <-- 152.1 <ND>
Total surface
34697 m2
Table 2: Total exchange surface of an absorption and desorption process
according to the pro-
cess embodying the invention including withdrawal of the solution stream which
has been pre-
heated in the heat transfer section (22a).
Heat ex- Heat output Stream Temp
Temp Stream Surface/
changer no. [kW] no. [ C] [ C] no.
LMTD WT [m2]
21 86271.2 11. 59.8 <-- 125.5 10. 14.2
12145
3. 41.1 - 115 6.
17 32454.4 11. 59.8 --> 35
12. 12.6 2568
<Kw> 37 <-- 19 <Kw>
8 33000 7. 125.4 --> 125.5 <> 26.6 1241
<ND> 152 <-- 152.1 <ND>
Total surface
18522 m2
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Table 3: Total exchange surface of an absorption and desorption process
according to the pro-
cess embodying the invention according to which the solution stream which has
been pre-
heated in the heat transfer section (22a) is completely routed to the top of
the stripping section
(22b).
Heat ex- Heat output Stream Temp Temp Stream
Surface/
changer no. [kW] . no. [ C] [ C] no.
LMTD WT [m2]
21 86271.2 11. 59.8 <-- 125.5 10. 14.2
12145
3. . 41.1 115 . 6.
17 32454.4 11. 59.8 -->
35 12. 12.6 2568
. <Kw> 37 <-- 19 <Kw>
8 37950 7. 124.7 --> 124.8 <> 27.3 1391
<ND > 152 <-- 152.1 < ND >
Total surface
18672 m2
[0034] By
the pre-heating of the stream (3), the initial temperature of the stream
(table 2,
11, 59.8 C) is considerably higher than without pre-heating the stream (3)
(table 1, 11, 46.1 C).
The comparison of the mean logarithmic temperature difference for heat
exchanger (21) shows
that the value according to table 2 is nearly half the value of table 1. This
means corresponding-
ly that the heat exchange surface required can be almost halved. A minor part
in the reduction
of the heat exchange surface is played by the fact that the heat transfer
output has been re-
duced by approx. 17%.
[0035] The results show that, by the mode of operation embodied by the
invention, it is
possible to save nearly 50% of the total heat exchange surface and one
complete heat ex-
changer in the absorption and desorption process. Based on the assumption that
the cost of
one square meter of heat exchange surface is approx. 500 Ã, it is possible in
this example to
save cost of approx. 8 million à as compared to prior art.
[0036] Table 3 shows the results for the process variant acc. to Fig. 3,
according to which
the stream which has been pre-heated in the heat transfer section (22a) is not
withdrawn from
the desorption device but is completely fed to the top of the stripping
section (22b). The overall
heat exchange surface required for this process variant is reduced in the same
way as for the
process variant according to Fig. 2. This is, however, to the detriment of a
significantly higher
amount of regeneration energy (37900 KW instead of 33000 KW). This corresponds
to an addi-
tional consumption of approx. 13% external heat energy, the procurement of
which involves
high cost. Therefore the process variant according to which the part-stream
pre-heated in the
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heat transfer section (22a) is withdrawn from the desorption device is clearly
more advanta-
geous than the operating mode according to which the stream remains in the
desorption device.
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List of reference numbers and designations:
1 Feed gas
2 Product gas
3 Laden solution stream
4 Laden part-stream
4a Pre-heated part-stream
4b Pre-heated part-stream
5a Laden residual stream
5b Laden, pre-heated merged stream
6 Pre-heated stream
7 Regenerated solution
8 Reboiler
9 Regenerated solvent stream
10 Regenerated solvent stream
11 Solvent stream after heat exchange
12 Cooled regenerated solution
13 Separated component
14 Cooled separated component
Reflux pump
16 Pump
17 Heat exchanger
18 Head condenser
19 Reflux drum
Absorption device
21 Heat exchanger
22 Desorption device
22a Heat transfer section
22b Stripping section
23 Pump
24 Branch
